This invention relates, in general, to heterojunction semiconductor devices, and more particularly, to semiconductor devices comprising silicon and carbon doped silicon.
Semiconductor heterojunction devices comprising at least two Group IV elements, silicon, germanium, and carbon, have been widely reported. In particular applications, heterojunction devices using Group IV elements are preferable over heterojunction devices using Group III and Group V elements for example, because heterojunction devices comprised of Group IV elements are more cost effective.
Carbon doped silicon devices have been widely reported. Carbon typically has been added to silicon in a variety of ways to provide a widened bandgap compared to silicon. A designer may vary the bandgap width by varying the concentration of the carbon in the carbon doped silicon material. Carbon has been added to silicon to form widened bandgap emitter regions in HBT devices. Silicon carbide is a widely used form of heavily carbon doped (&gt;40% carbon) silicon and has a bandgap of 2.2 eV to 3.3 eV compared to silicon's bandgap of 1.12 eV at 25.degree. C. Silicon carbide has been used to make wide bandgap field effect transistor devices that are more thermally, chemically, and mechanically stable than silicon devices and are more resistant to radiation damage than silicon devices.
Germanium-carbon doped silicon devices also have been reported. For example, germanium-carbon doped silicon also has been used for widened bandgap emitter regions in HBT devices. Germanium-carbon doped silicon devices have a disadvantage in that the concentrations of two elements, germanium and carbon, must be controlled in order to provide a quality semiconductor layer.
Germanium doped silicon devices also have been reported. Germanium has been added to silicon to provide a narrowed bandgap compared to silicon. However, germanmum doped silicon layers have several disadvantages. For example, in order for a sufficient narrowing to occur in the bandgap, relatively high concentrations, typically greater than 8% germanium, must be used. Since high concentrations of germanium are necessary to achieve bandgap narrowing, germanium doped silicon devices are susceptible to crystalline defects such as misfit dislocations. These crystalline defects severely limit the ability to produce narrowed bandgap devices.
Furthermore, the processes used to manufacture germanium doped silicon layers are costly. Chemical vapor deposition (CVD) techniques, such as molecular beam epitaxial deposition or conventional epitaxial deposition, typically are used to form the germanium doped silicon layers. These CVD techniques require significant capital investment and offer poor processing throughput. Also, using CVD techniques to selectively deposit germanium-doped silicon layers requires significant pre-deposition processing that adds to manufacturing costs. This increase in manufacturing cost limits the ability to integrate heterojunction devices with non-heterojunction devices within the same monolithic integrated circuit.
Although ion implantation has been used to dope silicon with germanium, large germanium implant doses are required to in order to achieve a narrowed bandgap characteristic. Large germanium implant doses require significant implanting time and result in significant damage to the silicon lattice making re-crystallization and defect reduction difficult.
Thus, there exists a need for a IV--IV semiconductor heterojunction device that has a narrowed bandgap compared to silicon, that uses relatively low concentrations of a group IV element to narrow the bandgap, that is less susceptible to defect formation, and that can be selectively formed using cost-effective manufacturing techniques.